In recent years, due to improvement of an image processing technique and an image forming technique, a copy of paper money obtained by a color copy machine is so exquisite that it is difficult to distinguish the copy from the actual paper money. Thus, a color copy machine has been proposed, in which a special original such as the paper money, which is prohibited from being copied by law, is detected and a copy operation for such a special original is prohibited from being carried out.
In an image recording apparatus disclosed, for example, in Japanese Laid Open Patent Application No.2-83571, image data obtained by reading of an original is compared with registered pattern data corresponding to a special original using a pattern matching method. It is then determined based on the comparison result whether or not the original is the special original (such as the paper money). When it is determined that the original is the special original, a copying operation for the original is prohibited or an image (an all black image) different from an original image is formed on a paper.
However, since there are limitations of image reading accuracy and image analyzing accuracy, it is difficult to determine, at a accuracy of 100%, whether or not an original set on the copy machine is the special original. Thus, a degree to which an image obtained by reading the original (an input image) is similar to the pattern data is evaluated as a similarity value. If the similarity value for the input image is equal to or greater than a predetermined reference similarity value, it is generally determined that the original to be copied is the special original.
In a case where it is determined, using the similarity value as has been described above, whether or an original set on the copy machine is the special original, if the original is clearly seen as being the special original, or if the original is clearly seen as being not the special original, it can be accurately determined whether or not the original is the special original or not. However, in a case of the original which is hardly seen as being the special original, such as a case of dirty paper money or a sample of securities, the determination result depends on the reference similarity value set in the copy machine. For example, if the reference similarity value is high, an original which should be determined as being the special original is easily determined as not being the special original. Thus, a case where the special original is illegally copied may occur. On the other hand, if the reference similarity value is low, an original which should be determined as being not the special original is easily determined as being the special original. Thus, a case where a copy process for a normal original is prohibited may occurs. In this case, the efficient copy process is deteriorated.
In addition, it has also been tried that input images are analyzed in detail and it is determined based on the analysis result whether or not an original corresponding to the input images is the special original. However, in this case, it takes a long time for the analysis of the input images, so that a processing speed is deteriorated when a normal original is copied.